As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the possible
issues with garbage collection I could think of and possible solutions
for them. This document is just a draft and only a proposal, critics and
comments are welcome.
Kind Regards
Benjamin Thaut
1) Preface
All modern Languages with fast and efficient GCs have build in support
for garbage collection within the compiler / virtual machine. Currently
the D language does have as much GC support as C++ has but fully relies
on the GC with a lot of language features and the standard library. As a
result the GC is slow, non parallel and always needs to stop all threads
before collection. This document suggests to build in better support for
garbage collection into the language so that creating a fast and
efficient GC becomes possible. A list of features that would be required
is described in this document.
2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the GC
can walk up the stack at any time in the program. The stack frame of
every function generated by the D compiler starts which a bitfield
(usually the size of a machine register) where each bit indicates that
these bytes are a pointer / reference. The bitfield needs to be large
enough to cover the whole stack frame of the function.
For example on x86: 11001...
1 = bytes 0 to 4 are a pointer
1 = bytes 4 to 8 are a pointer
00 = bytes 8 to 16 are not a pointer
1 = bytes 16 to 20 are a pointer
The last bit indicates whether the bitfield is continued in the
following bytes or not. 1 means continued 0 means finished.
Every scope generated by the D compiler would need additional code at
the start and end of the scope. When the scope is entered the bitfield
would be patched to represent the new variables inside the scope and
when the scope is left the bitfield is patched again to remove the
changes that were made on entering the scope.
Every time a function gets called that did not get generated by the D
compiler ( C / C++ etc functions) the compiler generates a call into the
runtime and passes the current stack pointer and stack base pointer to it.
void _d_externalCallStart(void* stackptr, void* baseptr);
Every time such a function returns the compiler generates a call into
the the runtime too.
void _d_externalCallEnd(void* stackptr, void* baseptr);
Every time a functions that can get called from other languages
(extern(C) etc) are executed the end callback is inserted at the start
of the functions and the start callback is inserted at the end of the
function.
Using these callbacks the GC can mark certain parts of the stack as
"non-D" and ignore them when scanning for bit fields and
references/pointers. It can just skip parts of the stack that are
"non-D" and therefore does not need a full stack frame within these
"non-D" sections.
All these features are required so that the GC can precisely scan
pointers/references on the stack and change them as necessary.
Remaining issues: The D compiler can freely move around value types on
the stack. With such move operations it would be necessary to fix up all
the bit fields. I needs to be investigated if this is doable.
3) Tracking references on the heap
For every class / struct a mixin template which is defined inside the
runtime gets instantiated. This template can then use introspection to
generate the necessary information to allow the GC to scan the pointers
within that struct / class precisely.
4) Thread local / global memory
A callback into the runtime needs to happen in the following cases:
- a __gshared variable is assigned
- a reference / pointer is casted to immutable
- a reference / pointer is casted to shared
void _d_castToGlobalMem(void* ptr);
This can be used by the GC to keep thread local pools of memory and move
a memory block to a global memory pool as soon as it is needed there.
5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a call
into the runtime and passes the new value of the reference / pointer
with it.
void _d_pointerChanged(void *ptr);
This can be used when writing a generational GC to have separate pools
for young and old generations. Every time the young generation needs to
be collected it can be avoided to scan the old generations pool because
it is sufficient to only check the pointers that have changed since the
last time the young generation was collected. With the above mentioned
callback it is easily possible to track these references.
Remaining issues:
-If the GC interrupts a thread right before any of the above mentioned
callbacks happen it will cause a invalid state for the GC and the GC
might access invalid pointers. It has to be investigated if this leads
to invalid behavior. It can be fixed by not interrupting a thread but
pause it the next time it calls any of callbacks, or other functions
that can be interrupted by the GC. This in turn could cause a thread to
be non pausable because it is stuck in a polling loop. The compiler
could identify loops without any GC interruptible function and manually
insert one.
-When pointers/references are passed inside processor registers the GC
cannot know if these values are actually pointers/references or
represent other values. If threads are paused at defined points in the
code as mentioned before this issues would be fixed because the state at
these points is known and can be handled accordingly.
6) Different interface for the GC
The current interface to the GC would have to change because the "this
block of memory might contain a pointer" approach wouldn't work anymore.
For example a block of memory and a delegate which iterates over all
pointers within the memory block could be used for user allocated memory
blocks. There should be a separate allocator function provided by the GC
that allocates memory that does not get moved around so it can be used
to pass it to non garbage collected code.
7) Compiler Options
Each of the above mentioned groups of features should be exposed as
compiler options so that you can turn them on/off depending on which
type of GC you use. Default on/off states for these features are set
within a config file depending on which type of GC currently ships per
default with druntime.
8) Conclusion
Garbage Collection brings a lot of advantages for the programmer using
the language but is not free and shouldn't be treated as free. Full
support for garbage collection is required to build a fast and efficient
GC. This additional support requires additional features within the
compiler but should result in a overall better performing language.

As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the
possible issues with garbage collection I could think of and
possible solutions for them. This document is just a draft and only
a proposal, critics and comments are welcome.

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.

This adds a lot of overhead to the runtime stack, esp. if you have deep
recursion. It's also not necessarily faster, since the GC now has to
parse a bitfield (a variable-length encoded bitfield, no less), instead
of just scanning words directly, which can be optimized by CPU-specific
microcode depending on the target platform.
[...]

Every scope generated by the D compiler would need additional code
at the start and end of the scope. When the scope is entered the
bitfield would be patched to represent the new variables inside the
scope and when the scope is left the bitfield is patched again to
remove the changes that were made on entering the scope.

This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.

Every time a function gets called that did not get generated by the
D compiler ( C / C++ etc functions) the compiler generates a call
into the runtime and passes the current stack pointer and stack base
pointer to it.
void _d_externalCallStart(void* stackptr, void* baseptr);
Every time such a function returns the compiler generates a call
into the the runtime too.
void _d_externalCallEnd(void* stackptr, void* baseptr);
Every time a functions that can get called from other languages
(extern(C) etc) are executed the end callback is inserted at the
start of the functions and the start callback is inserted at the end
of the function.
Using these callbacks the GC can mark certain parts of the stack as
"non-D" and ignore them when scanning for bit fields and
references/pointers. It can just skip parts of the stack that are
"non-D" and therefore does not need a full stack frame within these
"non-D" sections.

This may not be a bad idea, though it does introduce some overhead when
you cross language boundaries.

All these features are required so that the GC can precisely scan
pointers/references on the stack and change them as necessary.
Remaining issues: The D compiler can freely move around value types
on the stack. With such move operations it would be necessary to fix
up all the bit fields. I needs to be investigated if this is doable.

This can only make the GC slower, especially if it needs to update
variable-length encoded bitfields. Of course, you may be able to offset
this by making it possible to do real-time GC, (reduced throughput but
less waiting time for collection cycles) but that's a very complex
problem.

3) Tracking references on the heap
For every class / struct a mixin template which is defined inside
the runtime gets instantiated. This template can then use
introspection to generate the necessary information to allow the GC
to scan the pointers within that struct / class precisely.

So basically you're proposing a compacting precise-scanning GC instead
of the current conservative GC. There are pros and cons in either
approach; it'd be nice if you could compare them.
[...]

5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a
call into the runtime and passes the new value of the reference /
pointer with it.

This introduces a LOT of overhead, especially in a language like D which
manipulates a lot of pointers quite often (esp. if you use slices a
lot).
[...]

-If the GC interrupts a thread right before any of the above mentioned
callbacks happen it will cause a invalid state for the GC and the GC
might access invalid pointers. It has to be investigated if this leads
to invalid behavior. It can be fixed by not interrupting a thread but
pause it the next time it calls any of callbacks, or other functions
that can be interrupted by the GC.

This adds a lot of intermittent pauses in program execution.
The link I posted at the top has a GC implementation that doesn't
introduce *any* of this overhead (the GC runs concurrently with the
program), with no pause during a collection cycle (garbage is
incrementally collected when allocating new memory).
[...]

6) Different interface for the GC
The current interface to the GC would have to change because the
"this block of memory might contain a pointer" approach wouldn't
work anymore. For example a block of memory and a delegate which
iterates over all pointers within the memory block could be used for
user allocated memory blocks. There should be a separate allocator
function provided by the GC that allocates memory that does not get
moved around so it can be used to pass it to non garbage collected
code.

It would be nice if there was a way for GCs to be pluggable, especially
in the compiler. Currently, we can only swap GCs that implement the same
interface as the existing one, but to switch to a different GC model
like you're proposing would require a lot of compiler support.

7) Compiler Options
Each of the above mentioned groups of features should be exposed as
compiler options so that you can turn them on/off depending on which
type of GC you use. Default on/off states for these features are set
within a config file depending on which type of GC currently ships
per default with druntime.

This would be nice.

8) Conclusion
Garbage Collection brings a lot of advantages for the programmer
using the language but is not free and shouldn't be treated as free.

Full support for garbage collection is required to build a fast and
efficient GC. This additional support requires additional features
within the compiler but should result in a overall better performing
language.

I agree in principle, although for specific GC proposals, we'd need to
evaluate the pros and cons to determine what is improved and what
degrades. Unfortunately, GC is a complex problem, and different GCs work
better with different apps. I wouldn't be so quick to make claims about
the performance of GCs. It depends on what the app does.
T
--
EMACS = Extremely Massive And Cumbersome System

As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the
possible issues with garbage collection I could think of and
possible solutions for them. This document is just a draft and only
a proposal, critics and comments are welcome.

Yes I know about dgc it is better but still not on par with for example
the GC that is shipped with the .NET 4.0
All I'm saying is that without propper support from the compiler we are
not going to get GCs as good as in other modern languages.

[...]

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.

This adds a lot of overhead to the runtime stack, esp. if you have deep
recursion. It's also not necessarily faster, since the GC now has to
parse a bitfield (a variable-length encoded bitfield, no less), instead
of just scanning words directly, which can be optimized by CPU-specific
microcode depending on the target platform.

If you have a better idea for percise stack scanning I'm open for
suggestions.

[...]

Every scope generated by the D compiler would need additional code
at the start and end of the scope. When the scope is entered the
bitfield would be patched to represent the new variables inside the
scope and when the scope is left the bitfield is patched again to
remove the changes that were made on entering the scope.

This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.

Scopeds that don't have variables declared inside them don't need the
bitfield patching. so that argument is completely pointless. Scopes that
contain varaibles that are not pointers or refrences also don't need the
patching.

Every time a function gets called that did not get generated by the
D compiler ( C / C++ etc functions) the compiler generates a call
into the runtime and passes the current stack pointer and stack base
pointer to it.
void _d_externalCallStart(void* stackptr, void* baseptr);
Every time such a function returns the compiler generates a call
into the the runtime too.
void _d_externalCallEnd(void* stackptr, void* baseptr);
Every time a functions that can get called from other languages
(extern(C) etc) are executed the end callback is inserted at the
start of the functions and the start callback is inserted at the end
of the function.
Using these callbacks the GC can mark certain parts of the stack as
"non-D" and ignore them when scanning for bit fields and
references/pointers. It can just skip parts of the stack that are
"non-D" and therefore does not need a full stack frame within these
"non-D" sections.

This may not be a bad idea, though it does introduce some overhead when
you cross language boundaries.

All these features are required so that the GC can precisely scan
pointers/references on the stack and change them as necessary.
Remaining issues: The D compiler can freely move around value types
on the stack. With such move operations it would be necessary to fix
up all the bit fields. I needs to be investigated if this is doable.

This can only make the GC slower, especially if it needs to update
variable-length encoded bitfields. Of course, you may be able to offset
this by making it possible to do real-time GC, (reduced throughput but
less waiting time for collection cycles) but that's a very complex
problem.

3) Tracking references on the heap
For every class / struct a mixin template which is defined inside
the runtime gets instantiated. This template can then use
introspection to generate the necessary information to allow the GC
to scan the pointers within that struct / class precisely.

So basically you're proposing a compacting precise-scanning GC instead
of the current conservative GC. There are pros and cons in either
approach; it'd be nice if you could compare them.

I'm proposing a compacting percise-scanning generantional gc that has
thread local pools and can scan these thread local pools without
stopping the other threads. Also it will be able to collect young
generations without the need to scan the old generations.

[...]

5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a
call into the runtime and passes the new value of the reference /
pointer with it.

This introduces a LOT of overhead, especially in a language like D which
manipulates a lot of pointers quite often (esp. if you use slices a
lot).

I did not make this up, I know a smalltalk implementation that actually
does this and is pretty efficient.

[...]

-If the GC interrupts a thread right before any of the above mentioned
callbacks happen it will cause a invalid state for the GC and the GC
might access invalid pointers. It has to be investigated if this leads
to invalid behavior. It can be fixed by not interrupting a thread but
pause it the next time it calls any of callbacks, or other functions
that can be interrupted by the GC.

This adds a lot of intermittent pauses in program execution.

Why should there be pauses, there is just a additional check in every
callback to the gc there already is. When the gc wants to collect he
sets the pause flag to true and waits until all required threads paused
themselfs.

The link I posted at the top has a GC implementation that doesn't
introduce *any* of this overhead (the GC runs concurrently with the
program), with no pause during a collection cycle (garbage is
incrementally collected when allocating new memory).

Any non percise scanning algorithms will not be able to deal with memory
fragmentation and will also have uneccessary overhead for scanning
regions of memory that don't contain any pointers. Also they can leak
memory because some int value has the same value as a pointer and
therefore the gc does not free that block of memory.

[...]

6) Different interface for the GC
The current interface to the GC would have to change because the
"this block of memory might contain a pointer" approach wouldn't
work anymore. For example a block of memory and a delegate which
iterates over all pointers within the memory block could be used for
user allocated memory blocks. There should be a separate allocator
function provided by the GC that allocates memory that does not get
moved around so it can be used to pass it to non garbage collected
code.

It would be nice if there was a way for GCs to be pluggable, especially
in the compiler. Currently, we can only swap GCs that implement the same
interface as the existing one, but to switch to a different GC model
like you're proposing would require a lot of compiler support.

7) Compiler Options
Each of the above mentioned groups of features should be exposed as
compiler options so that you can turn them on/off depending on which
type of GC you use. Default on/off states for these features are set
within a config file depending on which type of GC currently ships
per default with druntime.

This would be nice.

8) Conclusion
Garbage Collection brings a lot of advantages for the programmer
using the language but is not free and shouldn't be treated as free.

There is no free GC. The only question is which trade offs you want to
make. Modern implementations like the .NET 4.0 garbage collector show
that all the things mentioned here are possible and are faster then
primitve implementations.

Full support for garbage collection is required to build a fast and
efficient GC. This additional support requires additional features
within the compiler but should result in a overall better performing
language.

I agree in principle, although for specific GC proposals, we'd need to
evaluate the pros and cons to determine what is improved and what
degrades. Unfortunately, GC is a complex problem, and different GCs work
better with different apps. I wouldn't be so quick to make claims about
the performance of GCs. It depends on what the app does.
T

Fully agree on this
I want to add that I did not make all this up. Most of the mentioned
features here are actually used in a Smalltalk implementation that
compiles Smalltalk to C for faster execution.

As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the
possible issues with garbage collection I could think of and
possible solutions for them. This document is just a draft and only
a proposal, critics and comments are welcome.

Yes I know about dgc it is better but still not on par with for
example the GC that is shipped with the .NET 4.0
All I'm saying is that without propper support from the compiler we
are not going to get GCs as good as in other modern languages.

I agree. Better compiler support would definitely be beneficial.
[...]

Every scope generated by the D compiler would need additional code
at the start and end of the scope. When the scope is entered the
bitfield would be patched to represent the new variables inside the
scope and when the scope is left the bitfield is patched again to
remove the changes that were made on entering the scope.

This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.

Scopeds that don't have variables declared inside them don't need
the bitfield patching. so that argument is completely pointless.
Scopes that contain varaibles that are not pointers or refrences
also don't need the patching.

That wasn't clear from your description. It makes more sense now.
[...]

5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a
call into the runtime and passes the new value of the reference /
pointer with it.

This introduces a LOT of overhead, especially in a language like D which
manipulates a lot of pointers quite often (esp. if you use slices a
lot).

I did not make this up, I know a smalltalk implementation that
actually does this and is pretty efficient.

OK.
[...]

-If the GC interrupts a thread right before any of the above
mentioned callbacks happen it will cause a invalid state for the GC
and the GC might access invalid pointers. It has to be investigated
if this leads to invalid behavior. It can be fixed by not
interrupting a thread but pause it the next time it calls any of
callbacks, or other functions that can be interrupted by the GC.

This adds a lot of intermittent pauses in program execution.

Why should there be pauses, there is just a additional check in
every callback to the gc there already is. When the gc wants to
collect he sets the pause flag to true and waits until all required
threads paused themselfs.

Whereas with a scheme like dgc there is no need for threads to pause at
all.

The link I posted at the top has a GC implementation that doesn't
introduce *any* of this overhead (the GC runs concurrently with the
program), with no pause during a collection cycle (garbage is
incrementally collected when allocating new memory).

Any non percise scanning algorithms will not be able to deal with
memory fragmentation

There are ways to deal with this. Though, granted, they're imperfect.

and will also have uneccessary overhead for scanning regions of memory
that don't contain any pointers.

True. But if the scanning is running in its own thread anyway, and no
other thread needs to wait for it, then this doesn't really matter, does
it?

Also they can leak memory because some int value has the same value as
a pointer and therefore the gc does not free that block of memory.

Yes, this is definitely a problem. It's not easy to fix this in a
language like D, though, without adding some overhead.
[...]

8) Conclusion
Garbage Collection brings a lot of advantages for the programmer
using the language but is not free and shouldn't be treated as free.

There is no free GC. The only question is which trade offs you want
to make. Modern implementations like the .NET 4.0 garbage collector
show that all the things mentioned here are possible and are faster
then primitve implementations.

True. But then again, D's GC is only a simple implementation. Just
because an advanced implementation of a GC beats D's GC doesn't
necessarily mean that that particular implementation's GC model is the
best.
But I'm not trying to argue against your proposal. I'm just saying we
should evaluate different GC models to see which one works best. But for
that we need a way to easily plug in different GC models, which requires
compiler support.
[...]

I agree in principle, although for specific GC proposals, we'd need
to evaluate the pros and cons to determine what is improved and what
degrades. Unfortunately, GC is a complex problem, and different GCs
work better with different apps. I wouldn't be so quick to make
claims about the performance of GCs. It depends on what the app does.
T

Fully agree on this
I want to add that I did not make all this up. Most of the mentioned
features here are actually used in a Smalltalk implementation that
compiles Smalltalk to C for faster execution.

The thing is, Smalltalk is different enough from D that it's hard to
draw conclusions about the performance of the GC when used in D just by
looking at its performance in Smalltalk. In Smalltalk, the programmer
doesn't have direct access to things like pointers and byte
representations of stuff. This allows the compiler to make optimizations
that would you couldn't safely do with a D program. It also means
programmers may implement the same algorithms differently in D than in
Smalltalk. So the memory usage patterns of a D program are quite
different from a Smalltalk program, and this will affect how a
particular GC behaves when applied to D.
Without actual testing we have no way to know for sure.
But regardless, we need better compiler support. No argument about that.
:)
T
--
Lottery: tax on the stupid. -- Slashdotter

If you have a better idea for percise stack scanning I'm open for
suggestions.

This is the problem with your proposal. It doesn't consider pro and cons
and actual data. It doesn't consider the alternatives. You go straight
to « How can we do that ? » without condidering « should we do that ? »
What would be the impact of being precise on the heap but not on the stack ?
1/ It would add some false positives. The future being 64bits, False
positive will be way less present than on 32bits machines. I did
searched for numbers on that, but couldn't found them. Considering this
is only on the stack, this may be neglectible (or not, but it
definitively require data).
2/ Data pointed by the stack are not movable.Again, what is the impact
of that. How much data could be promoted from young generation to old
one (considering we have young and old gen). How much data couldn't be
compacted ? What would the overhead on allocators ?
This definitively lack the required data and/or analysis of pro and cons.
Additionnaly, the stack is made like a linked list. Each function
calling another one register the return address. With this information,
we can have data about what is on the stack except for the very last
function called with no runtime overhead. This is another alternative.
But you have to consider that, even with a mask, you are not sure that
what is marked as a pointer is a pointer. A memory location can
represent different thing during a function execution. So thoses values
can only be considered as probable pointers, or we disable some compiler
optimizations. As we cannot be sure, the point 2/ stay valid.
Granted the overhead of the operation, it ay not worth it. To know that,
we need actual data on how much data is the stack is actually pointer,
and how much false positive we get. As the future is 64bits, I'm not
sure it is interesting for us.

Additionnaly, the stack is made like a linked list. Each function
calling another one register the return address. With this
information, we can have data about what is on the stack except for
the very last function called with no runtime overhead. This is
another alternative.

Yep. In one of my replies I considered the possibility of storing a
function ID on the stack, but that may not be necessary if the GC has
access to compile-time static info about each function, so just by
seeing the return address it knows which function it is, and can figure
out where the pointers are. (Of course there are other issues that need
to be addressed... but we can't decide on that without actual data.)

But you have to consider that, even with a mask, you are not sure
that what is marked as a pointer is a pointer. A memory location can
represent different thing during a function execution. So thoses
values can only be considered as probable pointers, or we disable
some compiler optimizations. As we cannot be sure, the point 2/ stay
valid.

I believe his proposal was for the function to manually update these
bits as it runs. It does introduce a lot of overhead. And like you said,
without actual hard data to show whether or not this overhead is
justified (offset by improved GC performance), how do we know that we
should do this at all? How do we know we aren't making it worse?

Granted the overhead of the operation, it ay not worth it. To know
that, we need actual data on how much data is the stack is actually
pointer, and how much false positive we get. As the future is 64bits,
I'm not sure it is interesting for us.

Actually, I believe David Simcha *is* considering the possibility of precise
scanning. But the proof is in the actual benchmarks. We don't know if it
will help or not unless we have real data to back it up.
T
--
Real Programmers use "cat > a.out".

As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the
possible issues with garbage collection I could think of and
possible solutions for them. This document is just a draft and only
a proposal, critics and comments are welcome.

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.

This adds a lot of overhead to the runtime stack, esp. if you have deep
recursion. It's also not necessarily faster, since the GC now has to
parse a bitfield (a variable-length encoded bitfield, no less), instead
of just scanning words directly, which can be optimized by CPU-specific
microcode depending on the target platform.
[...]

Every scope generated by the D compiler would need additional code
at the start and end of the scope. When the scope is entered the
bitfield would be patched to represent the new variables inside the
scope and when the scope is left the bitfield is patched again to
remove the changes that were made on entering the scope.

This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.

As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the possible
issues with garbage collection I could think of and possible solutions
for them. This document is just a draft and only a proposal, critics and
comments are welcome.
Kind Regards
Benjamin Thaut
1) Preface
All modern Languages with fast and efficient GCs have build in support
for garbage collection within the compiler / virtual machine. Currently
the D language does have as much GC support as C++ has but fully relies
on the GC with a lot of language features and the standard library. As a
result the GC is slow, non parallel and always needs to stop all threads
before collection. This document suggests to build in better support for
garbage collection into the language so that creating a fast and
efficient GC becomes possible. A list of features that would be required
is described in this document.
2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the GC
can walk up the stack at any time in the program.

I think walking up the stack to collect this info again and again (the
stack has a lot of "heavy frames" on the bottom, right?) sounds like a
tremendously slow way of getting necessary memory ranges. I'm no expert
though.
The stack frame of

every function generated by the D compiler starts which a bitfield
(usually the size of a machine register) where each bit indicates that
these bytes are a pointer / reference. The bitfield needs to be large
enough to cover the whole stack frame of the function.
For example on x86: 11001...
1 = bytes 0 to 4 are a pointer
1 = bytes 4 to 8 are a pointer
00 = bytes 8 to 16 are not a pointer
1 = bytes 16 to 20 are a pointer
The last bit indicates whether the bitfield is continued in the
following bytes or not. 1 means continued 0 means finished.
Every scope generated by the D compiler would need additional code at
the start and end of the scope. When the scope is entered the bitfield
would be patched to represent the new variables inside the scope and
when the scope is left the bitfield is patched again to remove the
changes that were made on entering the scope.

Again I'm no expert, but what happens when GC starts collecting a thread
stack amid this patching operation?

Every time a function gets called that did not get generated by the D
compiler ( C / C++ etc functions) the compiler generates a call into the
runtime and passes the current stack pointer and stack base pointer to it.
void _d_externalCallStart(void* stackptr, void* baseptr);
Every time such a function returns the compiler generates a call into
the the runtime too.
void _d_externalCallEnd(void* stackptr, void* baseptr);

Why would you need these? And if you start calling callback on every
operation, you may just as well pass direct ranges of memory to GC
without stack walk.
+ you can call D function from C one that in turn calls D one, think
extern(C) and callbacks.

Every time a functions that can get called from other languages
(extern(C) etc) are executed the end callback is inserted at the start
of the functions and the start callback is inserted at the end of the
function.
Using these callbacks the GC can mark certain parts of the stack as
"non-D" and ignore them when scanning for bit fields and
references/pointers. It can just skip parts of the stack that are
"non-D" and therefore does not need a full stack frame within these
"non-D" sections.
All these features are required so that the GC can precisely scan
pointers/references on the stack and change them as necessary.
Remaining issues: The D compiler can freely move around value types on
the stack. With such move operations it would be necessary to fix up all
the bit fields. I needs to be investigated if this is doable.
3) Tracking references on the heap
For every class / struct a mixin template which is defined inside the
runtime gets instantiated. This template can then use introspection to
generate the necessary information to allow the GC to scan the pointers
within that struct / class precisely.
4) Thread local / global memory
A callback into the runtime needs to happen in the following cases:
- a __gshared variable is assigned
- a reference / pointer is casted to immutable
- a reference / pointer is casted to shared
void _d_castToGlobalMem(void* ptr);
This can be used by the GC to keep thread local pools of memory and move
a memory block to a global memory pool as soon as it is needed there.
5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a call
into the runtime and passes the new value of the reference / pointer
with it.

Bye-bye any speed of p++ ? I mean I'm horrified, and I bet I'm not alone.

void _d_pointerChanged(void *ptr);
This can be used when writing a generational GC to have separate pools
for young and old generations. Every time the young generation needs to
be collected it can be avoided to scan the old generations pool because
it is sufficient to only check the pointers that have changed since the
last time the young generation was collected. With the above mentioned
callback it is easily possible to track these references.
Remaining issues:
-If the GC interrupts a thread right before any of the above mentioned
callbacks happen it will cause a invalid state for the GC and the GC
might access invalid pointers. It has to be investigated if this leads
to invalid behavior. It can be fixed by not interrupting a thread but
pause it the next time it calls any of callbacks, or other functions
that can be interrupted by the GC. This in turn could cause a thread to
be non pausable because it is stuck in a polling loop. The compiler
could identify loops without any GC interruptible function and manually
insert one.
-When pointers/references are passed inside processor registers the GC
cannot know if these values are actually pointers/references or
represent other values. If threads are paused at defined points in the
code as mentioned before this issues would be fixed because the state at
these points is known and can be handled accordingly.
6) Different interface for the GC
The current interface to the GC would have to change because the "this
block of memory might contain a pointer" approach wouldn't work anymore.
For example a block of memory and a delegate which iterates over all
pointers within the memory block could be used for user allocated memory
blocks. There should be a separate allocator function provided by the GC
that allocates memory that does not get moved around so it can be used
to pass it to non garbage collected code.
7) Compiler Options
Each of the above mentioned groups of features should be exposed as
compiler options so that you can turn them on/off depending on which
type of GC you use. Default on/off states for these features are set
within a config file depending on which type of GC currently ships per
default with druntime.

Combinatorial explosion of sets of options that doesn't necessary allow
a particular GC?

8) Conclusion
Garbage Collection brings a lot of advantages for the programmer using
the language but is not free and shouldn't be treated as free. Full
support for garbage collection is required to build a fast and efficient
GC. This additional support requires additional features within the
compiler but should result in a overall better performing language.

As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the possible
issues with garbage collection I could think of and possible solutions
for them. This document is just a draft and only a proposal, critics and
comments are welcome.
Kind Regards
Benjamin Thaut

7) Compiler Options
Each of the above mentioned groups of features should be exposed as
compiler options so that you can turn them on/off depending on which
type of GC you use. Default on/off states for these features are set
within a config file depending on which type of GC currently ships
per default with druntime.

Combinatorial explosion of sets of options that doesn't necessary
allow a particular GC?

Yeah, this is not a good way to go. Better would be for each GC come
with a GC description file, that describes what hooks/info it needs from
the compiler. Then the compiler can read this description file (passed
as a *single* compile flag) and do the right thing for that particular
GC. It can even automatically link in that particular GC without needing
you to specify anything further.
The GC description file can contain info like:
- Where/when to insert calls to GC functions (e.g., start collect cycle,
start mark cycle)
- Which function to use for allocating memory
- Any additional info required:
- e.g., map of each function's pointer local variables so that the GC
knows where the roots are;
- Whether or not hooks are needed for function entry/exit, and which
GC function to map them to;
- Any additional GC-specific info to insert into functions / structs /
etc..
- Whether or not pointer reads/writes need to include GC-specific code
(the description can include code to insert, if needed).
- Path to GC source(s) to be compiled into the program.
T
--
If Java had true garbage collection, most programs would delete
themselves upon execution. -- Robert Sewell

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.

[...]
I was thinking about this a bit more, and I had an idea: why bother with
storing bitfields on the stack? Any function's local pointer variables
are known at compile-time. So store a function ID (probably a pointer)
that maps to some static storage where this information is stored. Then
we always only need 1 word of extra storage on the stack frame, and the
GC can follow the pointer to get the info it needs. A recursively called
function won't incur the cost of duplicated copies of bitfields, its ID
points to same place. You can even have two different functions share
the same ID if they have pointer variables in exactly the same places.
The static storage can then be an array of relative stack offsets to the
function's pointer variables, so the GC can easily use this info to find
roots. No need to complicate the GC with manipulating bitfields, it's
just an int[].
If you want to get fancy, have the compiler reorder local variables so
that pointers are clustered together in blocks, then in the static
storage you can just encode pointer blocks by offset+length. (Although
this may not help much with (pointer,length) pairs on the stack, like
slices.) Or the compiler can reorder variables to maximize ID merges.
The same thing can be done for scopes, since their local variables are
also all known at compile-time.
T
--
Spaghetti code may be tangly, but lasagna code is just cheesy.

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.

[...]
I was thinking about this a bit more, and I had an idea: why bother with
storing bitfields on the stack? Any function's local pointer variables
are known at compile-time. So store a function ID (probably a pointer)
that maps to some static storage where this information is stored. Then
we always only need 1 word of extra storage on the stack frame, and the
GC can follow the pointer to get the info it needs. A recursively called
function won't incur the cost of duplicated copies of bitfields, its ID
points to same place. You can even have two different functions share
the same ID if they have pointer variables in exactly the same places.
The static storage can then be an array of relative stack offsets to the
function's pointer variables, so the GC can easily use this info to find
roots. No need to complicate the GC with manipulating bitfields, it's
just an int[].
If you want to get fancy, have the compiler reorder local variables so
that pointers are clustered together in blocks, then in the static
storage you can just encode pointer blocks by offset+length. (Although
this may not help much with (pointer,length) pairs on the stack, like
slices.) Or the compiler can reorder variables to maximize ID merges.
The same thing can be done for scopes, since their local variables are
also all known at compile-time.
T

But where would you know from which scope variables are still (or
already) valid and which are not?
void func()
{
void* ptr = gc.alloc(...);
//Ptr2, Ptr3 not valid yet
void* ptr2 = gc.alloc(...);
//ptr3 not valid yet
{
void* ptr3 = ptr1;
}
//ptr 3 not valid anymore
}
Also as the bitfiel is stored on the stack it will most likely ba
already in the cache. Whereas with your approach scanning 1 stackframe
would very likely also cause 1 cache miss because of the additional
indirection. So if you are scanning 30 stack frames it will cause 30
cache misses.
--
Kind Regards
Benjamin Thaut

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.

[...]
I was thinking about this a bit more, and I had an idea: why bother with
storing bitfields on the stack? Any function's local pointer variables
are known at compile-time. So store a function ID (probably a pointer)
that maps to some static storage where this information is stored. Then
we always only need 1 word of extra storage on the stack frame, and the
GC can follow the pointer to get the info it needs. A recursively called
function won't incur the cost of duplicated copies of bitfields, its ID
points to same place. You can even have two different functions share
the same ID if they have pointer variables in exactly the same places.
The static storage can then be an array of relative stack offsets to the
function's pointer variables, so the GC can easily use this info to find
roots. No need to complicate the GC with manipulating bitfields, it's
just an int[].
If you want to get fancy, have the compiler reorder local variables so
that pointers are clustered together in blocks, then in the static
storage you can just encode pointer blocks by offset+length. (Although
this may not help much with (pointer,length) pairs on the stack, like
slices.) Or the compiler can reorder variables to maximize ID merges.
The same thing can be done for scopes, since their local variables are
also all known at compile-time.
T

The break even point between bit-fields and pointers is 512 bytes. Although, if
one is thinking about on stack storage this probably doesn't matter since for
alignment purposes you'll always end up using at least 1 word if not 2.
However, a lot of functions use less then 512 (or 1024) bytes of of stack
space. I'd think it would be much more space efficient to have a separate
bitfield for the stack. Cache efficiency should be about the same as a on stack
representation, and scanning would, in theory, be quicker. IIRC, a separate
bit-field was the approach used by at least one precise C GC.

call into the runtime and passes the new value of the reference / =
pointer with it.

=20
void _d_pointerChanged(void *ptr);

D can call assembler, C routines like memset(), plain old opaque C =
library code, etc. What should the D compiler do in light of all the =
sources of memory changes that it can't monitor?

6) Different interface for the GC
=20
The current interface to the GC would have to change because the "this =

block of memory might contain a pointer" approach wouldn't work anymore. =
For example a block of memory and a delegate which iterates over all =
pointers within the memory block could be used for user allocated memory =
blocks. There should be a separate allocator function provided by the GC =
that allocates memory that does not get moved around so it can be used =
to pass it to non garbage collected code.
I posted a suggested new GC interface do the runtime mailing list 6 or =
so months ago. In short, I do think the current interface is lacking. =
Also, CDGC does support precise scanning and runs with Druntime. The =
big problem there is that CDGC is based on the Tango GC (where =
Druntime's GC originated) and someone needs to review all the GC changes =
since the Druntime project was created to see what may need to be merged =
into CDGC. I started on this once, but it turned out to be more work =
than I had time for.=

5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a
call into the runtime and passes the new value of the reference /
pointer with it.
void _d_pointerChanged(void *ptr);

D can call assembler, C routines like memset(), plain old opaque C
library code, etc. What should the D compiler do in light of all the
sources of memory changes that it can't monitor?

[...]
Yeah, the GC should be capable of dealing with non- safe code. Otherwise
it would just be too limited to be used for large non-trivial D
projects.
But still, some benchmarks do appear to be showing signs of a large
performance hit on the GC when there happens to be many integers that
look like valid pointers. This may be beyond the programmer's control,
since it could be the OS that gives the GC an address segment that just
happens to span integer values very commonly used throughout the app.
T
--
What doesn't kill me makes me stranger.

But still, some benchmarks do appear to be showing signs of a large
performance hit on the GC when there happens to be many integers that
look like valid pointers.

As I understand it, this is mitigated considerably on 64-bit platforms
due to the large pointer size.

[...]
True. You'd have to deliberately want to break the GC in order for
coincidental integer values to cause a significant problem.
But this problem could be a major issue if D was to become a significant
player on handhelds, which, if I understand correctly, are still largely
running 32-bit CPUs.
It's also a major problem on 16-bit platforms, but the only use of those
that I can conceive of are toy applications so it's probably not worth
the consideration. :)
T
--
That's not a bug; that's a feature!

4) Thread local / global memory
A callback into the runtime needs to happen in the following cases:
- a __gshared variable is assigned
- a reference / pointer is casted to immutable
- a reference / pointer is casted to shared
void _d_castToGlobalMem(void* ptr);
This can be used by the GC to keep thread local pools of memory and move
a memory block to a global memory pool as soon as it is needed there.

If we'd want per-thread mark/sweeping then shared memory must never own
unshared memory. I think this could be done using a separate allocator for
shared/immutable data. For casts this would require a transitive move of
the
data or it'd need to be prohibited.

4) Thread local / global memory
=20
A callback into the runtime needs to happen in the following cases:
- a __gshared variable is assigned
- a reference / pointer is casted to immutable
- a reference / pointer is casted to shared
=20
void _d_castToGlobalMem(void* ptr);
=20
This can be used by the GC to keep thread local pools of memory and move a=

memory block to a global memory pool as soon as it is needed there.

=20

If we'd want per-thread mark/sweeping then shared memory must never own
unshared memory. I think this could be done using a separate allocator for=

shared/immutable data. For casts this would require a transitive move of t=

he

data or it'd need to be prohibited.

Casting to/from shared needs a bit more logic anyway, so the proper thread f=
inalizes unshared objects. Casting away shared clears the block's owner and v=
ice versa. Nice perk to this is casting away shared could then detect a shar=
ing violation if a block already has another owner.=20=

You didn't mention what is the most important IMO.
In D, most data are thread local. Shared data are either shared or
immutable.
Both thread local data and immutable data lead to very interesting GC
optimisations. This is where we need language support.

2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the GC can
walk up the stack at any time in the program.

You say "every function needs a stack frame". Can you comment on this with
respect to leaf functions? No leaf function should ever generate a stack
frame, infact, typical leaf functions will never touch memory at all. This
is VERY IMPORTANT for critical loops. I have never worked on a project
where I did not depend on the performance of leaf functions to do the hard
work in the most critical parts of my application.
Obviously such functions would not be making allocations, and shouldn't be
interacting with the GC any way, so why is having a stack frame important?

The stack frame of every function generated by the D compiler starts which
a bitfield (*usually the size of a machine register*)...

Oh really? And how do we define that type? ;) (*cough* reference to
size_t/ptrdiff_t thread)

For example on x86: 11001...
1 = bytes 0 to 4 are a pointer
1 = bytes 4 to 8 are a pointer
00 = bytes 8 to 16 are not a pointer
1 = bytes 16 to 20 are a pointer
The last bit indicates whether the bitfield is continued in the following
bytes or not. 1 means continued 0 means finished.
Every scope generated by the D compiler would need additional code at the
start and end of the scope. When the scope is entered the bitfield would be
patched to represent the new variables inside the scope and when the scope
is left the bitfield is patched again to remove the changes that were made
on entering the scope.
Every time a function gets called that did not get generated by the D
compiler ( C / C++ etc functions) the compiler generates a call into the
runtime and passes the current stack pointer and stack base pointer to it.
void _d_externalCallStart(void* stackptr, void* baseptr);
Every time such a function returns the compiler generates a call into
the the runtime too.
void _d_externalCallEnd(void* stackptr, void* baseptr);

Can you comment on what those functions will actually do? It definitely
sounds very worrying to me to be turning every function call into THREE
calls.
Calling into extern code is certainly not rare... almost everything of any
use is an extern C lib. What about interaction with the OS? Trivial libs
like zlib? etc...
I wonder if there are alternative ways to detect a foreign stack. And I'm
not sure why it even matters, you can't depend on the extern ABI, how do
you unwind the stack reliably in the first place?

Every time a functions that can get called from other languages (extern(C)
etc) are executed the end callback is inserted at the start of the
functions and the start callback is inserted at the end of the function.
Using these callbacks the GC can mark certain parts of the stack as
"non-D" and ignore them when scanning for bit fields and
references/pointers. It can just skip parts of the stack that are "non-D"
and therefore does not need a full stack frame within these "non-D"
sections.
All these features are required so that the GC can precisely scan
pointers/references on the stack and change them as necessary.
Remaining issues: The D compiler can freely move around value types on the
stack. With such move operations it would be necessary to fix up all the
bit fields. I needs to be investigated if this is doable.
3) Tracking references on the heap
For every class / struct a mixin template which is defined inside the
runtime gets instantiated. This template can then use introspection to
generate the necessary information to allow the GC to scan the pointers
within that struct / class precisely.
4) Thread local / global memory
A callback into the runtime needs to happen in the following cases:
- a __gshared variable is assigned
- a reference / pointer is casted to immutable
- a reference / pointer is casted to shared
void _d_castToGlobalMem(void* ptr);
This can be used by the GC to keep thread local pools of memory and move a
memory block to a global memory pool as soon as it is needed there.
5) pointer / reference changed callback
Every time a pointer / reference is changed the D compiler emits a call
into the runtime and passes the new value of the reference / pointer with
it.
void _d_pointerChanged(void *ptr);
This can be used when writing a generational GC to have separate pools for
young and old generations. Every time the young generation needs to be
collected it can be avoided to scan the old generations pool because it is
sufficient to only check the pointers that have changed since the last time
the young generation was collected. With the above mentioned callback it is
easily possible to track these references.
Remaining issues:
-If the GC interrupts a thread right before any of the above mentioned
callbacks happen it will cause a invalid state for the GC and the GC might
access invalid pointers. It has to be investigated if this leads to invalid
behavior. It can be fixed by not interrupting a thread but pause it the
next time it calls any of callbacks, or other functions that can be
interrupted by the GC. This in turn could cause a thread to be non pausable
because it is stuck in a polling loop. The compiler could identify loops
without any GC interruptible function and manually insert one.
-When pointers/references are passed inside processor registers the GC
cannot know if these values are actually pointers/references or represent
other values. If threads are paused at defined points in the code as
mentioned before this issues would be fixed because the state at these
points is known and can be handled accordingly.
6) Different interface for the GC
The current interface to the GC would have to change because the "this
block of memory might contain a pointer" approach wouldn't work anymore.
For example a block of memory and a delegate which iterates over all
pointers within the memory block could be used for user allocated memory
blocks. There should be a separate allocator function provided by the GC
that allocates memory that does not get moved around so it can be used to
pass it to non garbage collected code.
7) Compiler Options
Each of the above mentioned groups of features should be exposed as
compiler options so that you can turn them on/off depending on which type
of GC you use. Default on/off states for these features are set within a
config file depending on which type of GC currently ships per default with
druntime.
8) Conclusion
Garbage Collection brings a lot of advantages for the programmer using the
language but is not free and shouldn't be treated as free. Full support for
garbage collection is required to build a fast and efficient GC. This
additional support requires additional features within the compiler but
should result in a overall better performing language.

I wonder if there are alternative ways to detect a foreign stack. And
I'm not sure why it even matters, you can't depend on the extern ABI,
how do you unwind the stack reliably in the first place?

[...]
This is a bit off-topic, but what happens in the current implementation
if you pass a D callback to a C function, and then throw an exception
from the callback? Does it work? Or does it do something really nasty?
T
--
Elegant or ugly code as well as fine or rude sentences have something in
common: they don't depend on the language. -- Luca De Vitis

I wonder if there are alternative ways to detect a foreign stack. And
I'm not sure why it even matters, you can't depend on the extern ABI,
how do you unwind the stack reliably in the first place?

[...]
This is a bit off-topic, but what happens in the current implementation
if you pass a D callback to a C function, and then throw an exception
from the callback? Does it work? Or does it do something really nasty?
T

This will work, but has serious drawbacks.
First of all, you are not sure the C function will release all resources
(free, fclose, etc . . .). So you cannot be sure of the state of your
program.
This is something that you don't want to do.

I wonder if there are alternative ways to detect a foreign stack. And
I'm not sure why it even matters, you can't depend on the extern ABI,
how do you unwind the stack reliably in the first place?

[...]
This is a bit off-topic, but what happens in the current implementation
if you pass a D callback to a C function, and then throw an exception
from the callback? Does it work? Or does it do something really nasty?

No, unless you consider a segfault to be really nasty. :)
Actually, it mostly works - i just tried it in a gtk app, and it works as
long as you catch the exception and only look at the error msg. If you don't
catch it (or try writeln(e) etc), then the result is something like:
action.Action!(int).Action.registerNS.MissingActionEx action.d(54): Action
"GUI" missing symbol 'int AppWindowClosed()'
----------------
./gtkapp() [0x8054366]
/usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x153052) [0xf7375052]
/usr/lib/i686/sse2/libgobject-2.0.so.0(g_closure_invoke+0x19b) [0xf71e25fd]
/usr/lib/i686/sse2/libgobject-2.0.so.0(+0x1ddc8) [0xf71f2dc8]
/usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit_valist+0x59a) [0xf71fa37f]
/usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit+0x34) [0xf71fa61f]
/usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x2a17f3) [0xf74c37f3]
/usr/lib/i686/sse2/libgtk-x11-2.0.so.0(gtk_main_do_event+0x8e6) [0xf7373856]
Segmentation fault
So something appears to get confused while walking the stack; another thing to
investigate later, i guess...
artur

This is a bit off-topic, but what happens in the current
implementation if you pass a D callback to a C function, and then
throw an exception from the callback? Does it work? Or does it do
something really nasty?

No, unless you consider a segfault to be really nasty. :)

Well, segfaults are nasty, but there are nastier things. :)

Actually, it mostly works - i just tried it in a gtk app, and it works
as long as you catch the exception and only look at the error msg. If
you don't catch it (or try writeln(e) etc), then the result is
something like:
action.Action!(int).Action.registerNS.MissingActionEx action.d(54): Action
"GUI" missing symbol 'int AppWindowClosed()'
----------------
./gtkapp() [0x8054366]
/usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x153052) [0xf7375052]
/usr/lib/i686/sse2/libgobject-2.0.so.0(g_closure_invoke+0x19b) [0xf71e25fd]
/usr/lib/i686/sse2/libgobject-2.0.so.0(+0x1ddc8) [0xf71f2dc8]
/usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit_valist+0x59a) [0xf71fa37f]
/usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit+0x34) [0xf71fa61f]
/usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x2a17f3) [0xf74c37f3]
/usr/lib/i686/sse2/libgtk-x11-2.0.so.0(gtk_main_do_event+0x8e6) [0xf7373856]
Segmentation fault
So something appears to get confused while walking the stack; another
thing to investigate later, i guess...